Initiation of an Immune Response
The reactivity of the immune system is finely tuned in order to control an over-reactivity to normal tissues (self) resulting in autoimmune or chronic inflammatory diseases. At the same time new structures, e.g. infectious agents, virus infected cells, cancer cells ( non-self) have to be recognised in order that an immune response is mounted. The initiation of a specific immune response is a well-orchestrated chain of events where multiple cell types interact. Antigen presenting cells take up, process and present peptides of a foreign antigen, in the context of the major histocompatibility complex (MHC), to lymphoid cells with unique receptors (T-cell receptor, TCR). These events culminate in the activation of effector functions such as the release of cytokines, production of specific antibodies and/or cellular cytotoxic activity.
The circulation of immune cells via the blood stream and lymphatics, from one peripheral (secondary) lymphoid tissue to another, and then to peripheral inflammatory sites, is a prerequisite, both for the initiation of the immune response and for efficient effector functions. Recirculation of immune cells and their recruitment to tissues are dependent on and regulated by adhesive interactions between these cells and vascular endothelial cells and extravascular connective tissue. These adhesive interactions are governed by the expression and activation of various adhesion molecules expressed on the cell membranes.
Integrins
Integrins are a superfamily of transmembrane glycoproteins found predominantly on leukocytes that mediate cell-cell and cell substratum interactions. They play an important role in the initiation and regulation of an immune response, tissue recruitment and migration of inflammatory cells and cytotoxic activity of lymphocytes. Integrins are heterodimers consisting of noncovalently associated α and β subunits. They are organized in subfamilies based on their β chain. The members of the β2 subfamily (LFA-1, Mac-1, p150,95 and αdβ2) have the same β subunit (CD18) but separate α subunits. All four molecules play a role in the inflammatory process. In addition, LFA-1 (Leukocyte Function Associated molecule-1, CD11a/CD18) is involved in adhesion of cytotoxic T cells to their target cells. Mac-1 (CR3, CD11b/CD18) is involved in phagocytosis and p150,95 (CD11c/CD18) is important for B cell activation. Their ligands consist of the cellular counter-receptors ICAM-1, ICAM-2 and ICAM-3 as well as fibrinogens, endotoxins, Factor X and the complement protein C3bi. Additional molecules with binding capacity to some of the β2 integrins are the soluble form of CD23 (low affinity receptor for IgE) and soluble CD16 (Fc γ receptor III). Although the integrins are constitutively expressed, they exist in a low-affinity state. A high-affinity state can be induced by clustering of the receptors or by a change in receptor conformation. Blockade of integrins will seriously influence initiation of an immune response, recruitment of inflammatory cell to tissues, migration of these cells within tissues and the cytotoxic activity of cells of the immune system.
Immunosuppression in Cancer
Malignant tumours manage to suppress immune mediated anti-tumour reactivity. The function of immune cells in cancer patients is thereby impaired. Generally this is more pronounced in tumour infiltrating mononuclear cells, TIMC, than in cells obtained from peripheral blood. It has for example repeatedly been demonstrated that the proliferative response to mitogens, such as phytohemagglutinin (PHA) or concanavalin A (ConA), is inhibited, natural killer cell (NK-cell) activity and cytotoxic activity of CTLs are reduced as is the maturation and function of dendritic cells and the immune balance seems to be directed to a T-helper 2 situation. Immunosuppression of TIMC can, however, at least to some extent be overcome in vitro, either by washing, preincubation before stimulation, or culturing in interleukin-2. Amazingly, the down regulation of the immune system, which relates to cancer, does not result in a seriously increased incidence of infectious diseases in these patients. Reasonably due to a regional systemic gradient of immunosuppressive agents.
Extracts or supernatants from tumours are often immunosuppressive. Several factors have been suggested to mediate this suppression, e.g., TGF-β, PGE2, IL-10, IL-4 and others, either being produced by the tumour cells as such or by tumour-infiltrating lymphocytes (TIL) or tumour associated macrophages (TAM). However, no fundamental mechanism has been identified so far.
During early stages, primary malignant tumours (or inoculates) can progress locally without giving rise to metastatic disease. This is compatible with the occurrence of regional immunosuppression with sustained systemic immune reactivity (so called concomitant immunity), indicating a regional—systemic gradient of immunosuppression. Systemic immunosuppression can thus be regarded as a systemic dissemination, or “spillover” of intra—tumoural suppression.
Immunosuppression in cancer patients appears already at an early stage: Immune parameters predicting the recurrence of radically operated renal cell carcinoma patients can be demonstrated already one week after primary radical surgery. Analysis of the sentinel node of breast cancer patients at primary surgery shows low numbers of dendritic cells (DC) and down-regulation of the zeta-chain of TCR. In addition, function parameters of circulating monocytes are down-regulated in stage II primary breast cancer patients.
The immunosuppression of cancer patients described above often involves an ongoing systemic, chronic inflammation with a pathological production of several cytokines, in particular IL-6 and TNF-α seems to be important mediators in this process. This results in a paraneoplastic syndrome with a poor performance status—impaired general condition, which is characterized by anorexia, fatigue, subfebrility and distortion of various biochemical laboratory parameters, e.g., low haemoglobin concentration, high numbers of platelets, increased numbers of blood monocytes, increased concentration of acute phase reactants, increased c-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) and other factors. For cancer patients this condition is correlated to the tumour burden of the patient, being worse in more advanced disease. In the clinical situation, attempts are often made to ameliorate the poor general condition of these patients by corticosteroid treatment.
Immune reactivity is normally down-regulated when the eliciting antigenic structures are eliminated. In progressive cancer, however, the opposite situation prevails as the antigenic structures of the tumour cells are not eliminated. The mechanisms for “cancer related” immunosuppression are still largely unknown.
Autoimmunity and Chronic Inflammation
From basic immunology it is known that the reactivity of the immune system is finely tuned (self-tolerance) in order to control an over reactivity to normal tissues otherwise resulting in autoimmune diseases. Tolerance to normal tissues is maintained by central eradication of “forbidden clones” and various mechanisms are active in maintaining peripheral tolerance.
Abnormalities in the induction or maintenance of self-tolerance thus lead to immune responses to self antigens and autoimmune diseases. These diseases, such as rheumatoid arthritis, multiple sclerosis and type I diabetes, count among the major medical problems of industrialized societies. Antibodies reactive with self-antigens, like DNA and immunoglobulin, as wells as T cells with reactivity towards self antigens, for example myelin basic protein, are found. Similar to the situation with progressive cancer, in autoimmune disease the antigenic structures (the self antigens) do not disappear. However, instead of a down regulation of the immune response in these diseases, the reactivity continues resulting in destruction of normal tissues. The etiology and immunoregulatory mechanisms of most autoimmune diseases remain unknown.
Therapeutic Possibilities in Cancer
Some malignant tumours, immunogenic tumours, can be recognised by the immune system as non-self and an immune response to these tumours is mounted. When such tumours start to grow progressively the immune control has been lost, but can in about 20 percent of these patients be reactivated by immunostimulatory treatment, e.g. interferon-alpha or interleukin-2.
As mentioned above, tolerance to normal structures is maintained by central eradication of “forbidden clones” and various mechanisms are active in maintaining peripheral tolerance, for example signalling via CTLA4 and regulatory CD4+CD25+ lymphocytes. Therapeutic strategies interfering with these mechanisms might result in some enhancement of anti-tumour reactivity but at the cost of an increased autoimmune reactivity.
Chronic inflammatory reactions in cancer patients often result in a poor response to the immunotherapy. There are some animal and human reports on the importance of the immune status of tumour bearers for response also to chemotherapy or radiotherapy. Immunostimulatory treatment of the dysregulated immune system of cancer patients might be counter-productive. If the immune system in cancer is directed to downregulation of the chronic inflammatory reaction there is a risk that further therapeutic immunostimulation will enhance the immunosuppression and thereby further downregulate the immune reactivity against the tumour cells. The strategy should therefore be to eliminate mediators of immunosuppression before the immune system is stimulated. In the present invention, such immunoregulatory factors are described. Strategies to minimise the pathological production or biological activity of such immunoregulatory factors being immunosuppressive in cancer patients include treatment with enzyme inhibitors, monoclonal antibodies or fragments thereof, synthetic constructs or signal transduction inhibitors.
Therapeutic Possibilities in Chronic Inflammatory or Autoimmune Disease
Therapy for autoimmune diseases consists mainly of anti-inflammatory drugs, particular corticosteroids and antibodies directed against inflammatory cytokines. In severe cases, immunosuppressive drugs, such as cyclosporin are used to block T cell activation. Plasmapheresis has also been used to reduce the levels of circulating antibodies or immune complexes. Thus, no therapy directed to the fundamental dysregulatory mechanism is available. The present invention describes factors, the lack of which will result in an uncontrolled activity of an inflammatory process. Administration of these factors to patients with chronic inflammatory or autoimmune disease will be of value to control the over-reactivity in these diseases.